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This article was downloaded by:[Society for Psychotherapy Research (SPR)] On: 6 December 2007 Access Details: [subscription number 762317397] Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Psychotherapy Research Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713663589 Meta-Analysis of Therapist Effects in Psychotherapy Outcome Studies Paul Crits-Christoph a; Kathryn Baranackie a; Julie Kurcias a; Aaron Beck a; Kathleen Carroll b; Kevin Perry c; Lester Luborsky a; A. McLellan a; George Woody a; Larry Thompson d; Dolores Gallagher d; Charlotte Zitrin e a University of Pennsylvania. b Yale University. c Northwestern University. d Stanford University School of Medicine. e Long Island Jewish Hillside Medical Center. Online Publication Date: 01 October 1991 To cite this Article: Crits-Christoph, Paul, Baranackie, Kathryn, Kurcias, Julie, Beck, Aaron, Carroll, Kathleen, Perry, Kevin, Luborsky, Lester, McLellan, A., Woody, George, Thompson, Larry, Gallagher, Dolores and Zitrin, Charlotte (1991) 'Meta-Analysis of Therapist Effects in Psychotherapy Outcome Studies', Psychotherapy Research, 1:2, 81 - 91 To link to this article: DOI: 10.1080/10503309112331335511 URL: http://dx.doi.org/10.1080/10503309112331335511 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 Psychotherapy Research l ( 2 ) 81-91, 1991 META-ANMYSIS OF THERAPIST EFFECTS IN PSYCHOTHERAPY OUTCOME STUDIES Paul Crits-Christoph, Kathryn Baranackie, Julie S. Kurcias, Aaron T. Beck University of Pennsylvania Kathleen Carroll Yale University Kevin Perry Northwestern University Lester Luborsky, A. Thomas McLellan, George E. Woody University of Pennsylvania Larry Thompson, Dolores Gallagher Stanford University School of Medicine Charlotte Zitrin Long Island Jewish Hillside Medical Center In a meta-analysis, we examined factors that could account for the differences in therapist efficacy evidenced in psychotherapy outcome studies. The factors investigated were: ( 1) the use of a treatment manual, (2) the average level of therapist experience, (3) the length of treatment, and (4) the type of treatment (cognitivehehavioral versus psychodynamic). Data were obtained from frfteen psychotherapy outcome studies that produced 27 separate treatment groups. For each treatment group, the amount of outcome variance due to differences between therapists was calculated and served as the dependent variable for the meta-analysis. Each separate treatment group was coded on the above four variables, and multiple regression analyses related the independent variables to the size of therapist effects. Results indicated that the use of a treatment manual and more experienced therapists were associated with small differences between therapists, whereas more inexperienced therapists and no treatment manual were associated with larger therapist effects. The findings are discussed in terms of the design and the analysis of psychotherapy outcome research. Preparation of this manuscript was supported in part by National Institute of Mental Health Grant MH-40472 and NIMH Career Development Award MH-00756to Paul Crits-Christoph.The authors wish to acknowledge the generosity of the investigators who lent us their data, including David Barlow, Tom Borkovec, Tim Brown, Joseph Collins, Robert DeReubeis, Irene Elkin and Principal Investigators of the NIMH Collaborative Treatment of Depression Program (John Watkins, Stuart Sotsky, Stan Imber), Steve Hollon, Bernard Liberman, Paul Pilkonis, and William Piper. Address correspondence to Paul Crits-Christoph,Ph.D. Hospital of the University of Pennsylvania 308 Piersol Building, 3400 Spruce Street Philadelphia, PA 19104-4283. 81 Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 82 CRITS-CHRISTOPH ET AL. The therapist’s contribution to psychotherapy outcome has long been a topic of concern among clinicians and researchers (Beutler, Crago, & Arizmendi, 1986; Gurman & Razin, 1977). Recently, reports of differences between therapists in their patients’ outcomes have been appearing (Luborsky, McLellan, Woody, O’Brien, & Auerbach, 1985, Luborsky et al., 1986;see review by Lambert, 1989). The statistical implications of such “therapist effects” for conducting comparative studies of psychotherapies have been detailed by Crits-Christoph and Mintz (1991). These authors also observed a wide variation in the magnitude of therapist effects across a number of studies. We report here the results of a meta-analysis attempting to describe and to account for some of the variability in therapist effects across studies. Although a variety of factors might be hypothesized to account for why some studies evidence larger therapist effects than others, we chose to study four particular factors. Because of the need, given the sample size, to restrict the number of independent variables, only four factors were studied. The four factors included were treatment type, length of treatment, use of a treatment manual, and experience level of the therapists. We describe the rationale for our choice of each of these factors and our hypotheses about them, below. Of primary interest for our meta-analysiswas the question of whether the use of a treatment manual leads to smaller therapist effects. The original justification for the implementation of treatment manuals in psychotherapy outcome research was based upon the need to standardize the delivery of treatment techniques (Luborsky & DeRubeis, 1984). To the extent that techniques produce therapeutic change, it follows that making the delivery of such techniques uniform across therapists should reduce differences between therapists in their outcomes. Studies that did not employ treatment manuals, therefore, should be on the average more prone to yield therapist effects. Experience level of therapists is an obvious variable to explore in terms of its relevance to therapist effects. In fact, Perry and Howard (1989) recently reported data from three samples where the size of therapist effects may have been a function of differential experience levels. It might be expected that a sample of inexperienced therapists (e.g., psychology interns or psychiatric residents) would contain some ineffective therapists. These therapists might improve with further training, or they might choose not to pursue careers as therapists after being disappointed by their results. A sample of inexperienced therapists might also contain some naturally gifted therapists, and thus variability within such a sample could be expected. On the other hand, a group of experienced therapistsespecially those who participate in research studies-might be expected to be more uniform in their delivery of treatment. Type of treatment is an additional factor that could be related to the size of therapist effects. In more structured therapies (e.g., cognitive or behavioral treatments), where the treatment approach is well defined, there may be less opportunity for the therapist to stray off course. Psychodynamic therapies, in contrast, generally do not spec@ or mandate precise therapist behaviors; therefore, the dynamic therapists may have more leeway to go in different directions. The complexity of the theory and clinical material in psychodynamic therapy may also require more skill and/or produce more varied therapist interpretations and responses. Fewer therapists may be proficient at these complex skills, and therefore larger differences between therapists would be expected for psychodynamic treatment. Length of treatment is the final factor considered as a predictor of therapist Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 META-ANALYSIS O F THERAPIST EFFECTS 83 effects. We hypothesized that short-term,goal-directed therapies, regardless of the treatment orientation, would be expected to show more uniform outcomes across therapists. In a longer-term treatment it might be expected that the therapist’s skill or personality would have more positive or negative effects on patients; increased variability among therapists would result. METHOD SAMPLE Data were obtained from the following treatment studies for inclusion in the meta-analysis: Woody, McLellan, Luborsky, and O’Brien (1990) Beck, Hollon, Young, Bedrosian, and Budenz (1985), and Rush, Beck, Kovacs, and Hollon ( 1977) (data from these two studies were combined) Pilkonis, Imber, Lewis, and Rubinsky (1984) Hollon et al. (1983) Piper, Debbane, Bienvenu, and Garant (1984) Thompson, Gallagher, and Breckenridge ( 1987) Klein, Zitrin, Woerner, and Ross (1983) Luborsky and Crits-Christoph ( 1988) Nash et al. (1965) Borkovec and Mathews (1988) Perry and Howard ( 1989) Carroll, Rounsaville, and Gawin ( 1990a) Carroll, Rounsaville, and Gawin ( 1990b) Elkin et al. (1989) Barlow, Craske, Cerny, and Klosko (1989) Many of the above studies included comparisons between different forms of treatments or different lengths of treatments. Because we were interested in the relationship of these factors to the size of therapist effects, we included each treatment group as a separate unit for the meta-analysis. We were, however, concerned with potential nonindependence resulting from the same therapists treating patients in different treatment conditions within a study. For studies where therapists were crossed with treatment condition, and the treatment conditions were not distinguishable on our independent variables (i.e., all treatment conditions involved the same form of therapy and same length of treatment), we either selected one of the treatment conditions for our meta-analysis, or examined the therapist main effect (plus therapist by treatment interaction) across treatment conditions. For studies that involved a comparison of psychotherapy plus medication with psychotherapy alone, or psychotherapy plus placebo, we selected the psychotherapy alone or psychotherapy plus placebo condition, since a combined medication plus psychotherapy condition might obscure potential differences between psychotherapists. The above 15 studies produced a total of 27 treatment groups and a total of 141 therapists. Each of the treatment groups is described briefly below. All studies involved the treatment of outpatients. Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 84 CRITS-CHRISTOPH ET AL. In data available to date from the Woody et al. (1990) study, 30 methadonemaintained, opiate-dependent patients were treated by three therapists using dynamic psychotherapy. We combined the Beck et al. (1985) and Rush et al. (1977) studies. Of the 37 depressed patients treated with cognitive therapy alone, we used for our analysis only those therapists who treated at least two treatment-completing patients. This left a total of eight therapists who treated 23 patients. The Pikonis et al. (1984) investigation yielded three treatment groups: individual ( n = 22); conjoint ( n = 21); and group ( n = 21). These were comprised of mixed diagnosis patients who were treated by nine therapists, three for each treatment modality. From the Hollon et al. (1983) study, we examined the data from the cognitive therapy condition alone. This consisted of four therapists who provided cognitive therapy to a total of 16 depressed patients. The Piper et al. (1984) study supplied us with four groups: short-term individual therapy ( n = 21); long-term individual therapy ( n = 20); short-term group therapy ( n = 19); and long-term group therapy ( n = 19). The patients in this study were all suffering from neurotic or mild-to-moderate characterological problems. Three therapists, all psychoanalytically oriented, treated between five and eight patients with each therapy modality. We analyzed the data from the three treatment groups of the Thompson et al. ( 1987) study. The behavioral therapy group consisted of three therapists treating 2 5 patients. There were 27 patients in the cognitive therapy group who were treated by four therapists. In the brief psychodynamic therapy group, three therapists treated 24 patients. The patients were all elderly people with a diagnosis of major depression. The Klein et al. (1983) study examined the treatment ofphobias (agoraphobia, simple phobia, and mixed phobia). From this study we examined the data from the behavior therapy plus placebo (n = 67) condition. Five therapists participated. The data from the psychodynamic treatment group of the Luborsky and CritsChristoph (1988) study were analyzed. A total of 23 depressed patients were treated by four therapists. The Nash et al. ( 1965) project included two conditions (dynamic therapy with and without a role induction interview), and a total of 40 patients. Since the same four therapists treated patients in both conditions and the conditions were the same in regard to our independent variables (i.e., treatment type, length, and use or nonuse of a manual), we analyzed the data as a whole, calculating the main effect for therapist and condition by therapist interaction. The Borkovec and Mathews ( 1988) study compared three treatment groups (nondirective therapy, coping desensitization, and cognitive therapy; all groups also included relaxation therapy) in the treatment of generalized anxiety disorder and panic disorder. The same four therapists were used in all three conditions. Thus, for this study we also calculated the main effect for therapists across conditions and the therapist by treatment interaction. The Perry and Howard (1989) investigation provided data on five treatment groups. Patients were mixed diagnosis outpatients treated with psychodynamic psychotherapy. One group of 80 patients was treated by 15 highly experienced therapists. A second group of 45 patients was treated by 8 moderately experienced therapists. A third group of 30 patients was treated by 9 highly experienced therapists. The fourth group of 40 patients was treated by 13 moderately experienced therapists; the fifth group consisted of 6 inexperienced therapists who treated 17 patients. The Carroll, Rounsaville, and Gawin ( 1990a) study Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 META-ANALYSIS OF THERAPIST EFFECTS 85 included four conditions in the treatment of cocaine abuse: a cognitive-behaviorallyoriented relapse prevention versus clinical management, crossed with medication versus placebo. We selected the relapse prevention plus placebo condition ( n = 18 patients treated by 4 therapists) for inclusion in our meta-analysis.A second study by Carroll, Rounsaville, and Gawin (1990b) on treatment of cocaine abuse included an interpersonal psychotherapy condition. Two therapists treated a total of 2 1 patients. For the Collaborative Treatment of Depression study (Elkin et al., l989), data were available on 7 cognitive-behavioraltherapists who treated 36 completer patients and 9 interpersonal therapists who treated 46 completer patients. The Barlow et al., (1989) study included a total of 36 patients with panic disorder, and involved three treatment conditions: cognitive therapy plus exposure, relaxation therapy, and a combined cognitive plus exposure and relaxation condition. The same five therapists treated patients in all conditions, and therefore the data from this study were analyzed as a whole. MEASURES The dependent variable of interest was the size of the therapist effect for each treatment group. For each outcome measure within each separate group, we performed a one-way analysis of variance (or covariance when a pretreatment score was appropriate to use as a covariate), specfying therapist as a random factor. From the equations for expected mean squares we calculated the variance component for the therapist factor, and divided it by total variance to obtain a percent of variance explained by the therapist factor. For the Borkovec and Mathews (1988), Nash et al. (1965), and Barlow et al. ( 1989) studies, which were not broken down into separate conditions, we performed two-way ANOVAs with treatment condition and therapist as factors. With these studies we combined the therapist main effect and therapist by treatment interaction in order to obtain an overall index of therapist contribution to outcome. The number of separate, available outcome measures within each study ranged from 1 to 18. The percent of variance due to therapist was averaged across all outcome measures within each study. In addition, we retained the highest value for each study as a measure of the largest therapist effect demonstrated, since the average might tend to obscure therapist effects on certain measures. The four independent variables were coded on each treatment group as follows. For type of treatment, we created a dichotomous variable designating all cognitive or behavioral treatments as one type, and all psychodynamic or interpersonal treatments as another. Similarly, a dichotomous variable was coded for the use versus the nonuse of a treatment manual in the study. For therapist experience, we created a three-level variable: ( 1) limited experience (psychology intern, psychiatric resident, or less than one year of postdoctoraVpostresidency experience), ( 2 ) moderate experience (1 to 5 years of postdoctoral or postresidency experience, and (3) extensive experience (greater than 5 years postdoctoral or postresidency experience). The average level of experience of the therapists for a given treatment group was coded into one of the above categories. Note that these experience levels refer to total experience performing psychotherapy, not experience with a given population under study (e.g., depressed patients). The final independent variable was length of treatment. The score for this was the logarithm of the average number of treatment sessions completed by each group of patients. CRITS-CHRISTOPH ET AL. Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 86 RESULTS Descriptive Characteristicsof Treatment Groups. Table 1 presents descriptive information about the 27 treatment groups on the four independent variables, as well as on the measures of size of therapist effects. Most notable is the variability across treatment groups on the size of therapist effects. Some studies showed large differences (up to 48.7% of the outcome variance) between therapists,while others show no differences. This large variability allowed us to proceed with our attempt to explain the variability from characteristics of the studies. The average treatment group exhibited a therapist effect of 8.6% of the outcome variance (mean across outcome measures). This is close to Cohen’s (1969) definition (ix., 9 % ) of a medium-size effect for the behavioral sciences. A treatment manual was employed for 48% of the treatment groups. Two-thirds of the treatments were psychodynamic, and most (59% ) of the studies used highly experienced therapists. There was, however, sufficient range on these variables to allow testing their relationship to the size of therapist effects. The average treatment lasted about 27 sessions. Intercorrelations Among Independent Variables. The intercorrelations among the four independent variables are presented in Table 2. As can be seen, a substantial correlation ( r = -.75) between the use of a treatment manual and length of treatment was evident (the manual guided therapies tended to have fewer sessions). In addition, treatment type was moderately correlated ( r = .58) with use of a manual (the psychodynamic therapies less often involved the use of a formal treatment manual). Experience level of the therapists was relatively uncorrelated with the other factors. Table 1. Descriptive Data on 27 Treatment Groups % Percent of Variance due to therapist Average across measures Mean SD Range Selecting largest effect Mean SD Range Use of Treatment Manual yes no Length of Treatment (number of sessions) Mean SD Experience of therapists Inexperienced (less than 1 year) Moderately experienced (1-5 years) Highly experienced (greater than 5 years) Type of treatment Cognitive/Behavioral Psychodynamic 8.6 10.1 0 - 48.7 22.3 19.8 0-72.9 48.1 51.9 27.6 24.2 14.8 25.9 59.3 33.3 66.7 Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 META-ANALYSIS OF THERAPIST EFFECTS 87 Table 2. Intercorrelations Among Independent Variables Length Manual Treatment type ( 1 = Dynamic, 2 = Cognitive/ Behavioral) Use of manual 1 = no, 2 = yes) Length of treatment (log of sessions) .58" Therapist Experience -.42' .oo -.75** -.08 .31 *p < .05.**p < .01. AU tests are two-tailed. The high correlation between use of a manual and length of treatment makes it difficult to unravel the unique effects of these factors. To avoid multicollinearity problems in multiple regression analyses, we selected the treatment manual variable for inclusion, rather than length of treatment, since it was of primary interest to us. Prediction of Size of Therapist Effects. Simple correlations were calculated between the four independent variables and the two measures of the size of the therapist effect (average and largest therapist effect within each treatment group). As can be seen in Table 3, both use of a treatment manual and therapist experience evidenced statistically significant correlations with the average therapist effect. A trend 0, < .lo) for type of treatment was also apparent. For the largest therapist effect, type of treatment, use of a manual, and length of treatment showed significant correlations. All relationships were in the predicted direction. The results of a multiple regression analysis entering treatment type, use of a manual, and therapist experience are presented in Table 4. Partial correlations of each independent variable with the two criteria are given. The partial correlations with average therapist effect revealed that both use of a manual and therapist experience remain as statistically significant independent predictors. Use of a treatment manual also produced a sizeable ( r = -.50) partial correlation with the largest therapist effect criterion. The findings for type of treatment, however, disappeared when we controlled for the overlap between it and use of a manual. These three predictors jointly explained 4 1% of the variance in the average therapist effects across studies (adjusted R squared = .33), and 42% of the variance in the largest therapist effect (adjusted R squared = .34). Table 3. Relationships of Treatment Type, Length, Use of Manual, and Therapist Experience Level to Average Therapist Effect and Largest Therapist Effect in 27 Treatment Groups: Simple Correlations Predictor Average Effect Treatment type ( 1 = Dynamic, 2 = CognitiveBehavioral) Use of manual ( 1 = no, 2 = yes) Therapist experience Treatment length *p < .lo. **p< .05.***p< .01. All tests are two-tailed Largest Effect -.33' -.41" -.45" -. 58' '' -.41" .28 -.24 .39" CRITS-CHRISTOPHET AL. Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 88 Table 4. Relationship of Treatment Type, Use of Manual,and Therapist Experience Level to Average Therapist Effect and Largest Therapist Effect in 27 Treatment Groups: Partial Correlations Predictor Average Effect Largest Effect Treatment type ( 1 = Dynamic, 2 = CognitiveBehavioral) -.09 -.I0 Use of manual (1 = no, 2 = yes) -.42*' -.50* * Therapist experience -. 50" -.34' *p < .10 **p< .05. ***p< .01. All tests are two-tailed DISCUSSION One important finding in our meta-analysis was that differences between therapists in outcome were, on the average, at the level of a medium effect size. Wide variability across studies, however, was apparent, rendering our analysis of the factors contributing to the size of the therapist effects more important. Summarizing our predictive findings, the factors most highly related to the size of therapist effects were the use of a treatment manual and the experience level of therapists. These variables gave the highest partial correlations with the largest therapist effect and the average therapist effect, respectively. As mentioned, however, the overlap among the four predictor variables makes it difficult to separate the independent effects of specific variables. In addition to the confounding of independent variables, a variety of other limitations of this meta-analysis must also be mentioned to put the findings in context. For one, the relatively small sample size (N = 27 treatment groups) limited the statistical power of the analysis. Secondly, our collapsing the treatment types into two broad categories (cognitive-behavioral versus psychodynamic) may have obscured potential effects of a more fine-grained analysis of treatment type. The cognitive-behavioral category, for example, ranged from a Beck cognitive therapy approach, through relapse prevention and exposure treatment. In addition, our results say nothing about the factors related to therapist effects in forms of psychotherapy other than cognitive-behavioral and psychodynamic. A third factor to consider in interpreting the results is the potential role played by type of outcome measure. We analyzed the results across all available measures without any attention to differences among types of measures. In addition, for some studies, a large number of outcome measures were available; for other studies, only one outcome measure was available. Other aspects of the studies not examined here, such as characteristics of the patient sample, may be important to the size of therapist effects. Certain patient diagnoses may respond to treatment more uniformly. Some patient diagnoses, such as antisocial personality, may not respond at all to psychotherapy (Woody, McLellan, Luborsky, & O'Brien, 19S5), and therefore differences between therapists will not emerge, since all therapists may achieve poor results. Moreover, therapist experience may interact with patient characteristics. For example, Downloaded By: [Society for Psychotherapy Research (SPR)] At: 13:57 6 December 2007 META-ANALYSIS O F THERAPIST EFFECTS 89 with difficult patients, such as those with a borderline personality disorder, therapist experience may be crucial. On the other hand, in the treatment of simple phobias with a behavioral approach, uniformly good outcomes may be attainable regardless of therapist experience. Number of years of therapist experience in working with a particular population (e.g., elderly depressed patients) may be more important than total amount of experience, as was measured in this metaanalysis. Assuming the findings reported here are reliable, the results for use of a treatment manual and therapist experience have implications for the design of future research studies, as well as for the interpretation of published studies. These data lend support to the movement in the psychotherapy research field towards the standardization of the treatment variable through the use of treatment manuals and the use of experienced therapists trained to performance criteria prior to the start of the studies. These procedures appear to achieve their goal of reducing variability due to differences among therapists. In terms of the interpretation of published studies, the concerns raised by Crits-Christoph and Mintz ( 199 1 ) regarding the statistical implications of therapist effects for conducting comparative outcome studies seem to be especially relevant to the older treatment outcome studies that were less well controlled (i.e., less experienced therapists were used and treatment manuals were not employed). In particular, Crits-Christoph and Mintz ( 1991) argue that by ignoring the therapist factor, some investigators may have reported differences between treatments that were actually a function of therapist differences. More recent comparative outcome studies that use experienced therapists and treatment manuals are less likely to have large therapist effects, and therefore the results are less likely to be in question because of unanalyzed therapist effects. Despite this tendency for the better-controlled, recent comparative studies to have smaller therapist effects, the suggestion of Crits-Christoph and Mintz ( 199 1 ) that investigators routinely perform preliminary analyses for the presence of therapist effects is still germane. The current meta-analysis is only preliminary, and our understanding of the circumstances that produce differences between therapists is far from complete. The success of the treatment manuals and the use of experienced therapists in reducing differences between therapists, while important to comparative outcome studies, does not necessarily suggest that all research studies should implement these types of controls. Rather, the use of these procedures to reduce therapist variability would depend upon the particular research question at hand. In fact, for many purposes, it may be advisable to maximize therapist differences. If, for example, an investigator is interested in the relationship of the quality or quantity of the therapist’s technique to the outcome of treatment, a less standardized sample of therapists may be necessary to test the hypothesis adequately. The use of experienced, manual-guided therapists may restrict the range of the technique variable of interest and prevent the uncovering of a relationship with outcome. Further studies of the conditions that produce differences between therapists (e.g., Perry & Howard, 1989) also should not be restricted to the manualguided methodology that has otherwise become the standard in the field. A better understanding of the psychotherapeutic process can come from both controlled trials of standardized treatment packages, and more naturalistic investigations of the differences between therapists in the process and the outcomes of their treatments. 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